1. Field of the Invention
The invention is generally related to offshore platforms fabricated from tubular members, and more particularly to a method of construction that reduces the impact of seawater pressure on the design of tubular members submerged in seawater.
2. General Background
As is well known to those practiced in the art, the structural members of an offshore platform which are immersed in seawater usually are tubulars and must be designed to resist net external hydrostatic pressure in combination with whatever other loads are imposed on the structural members. The structures normally must be designed to float so that they can be installed using controlled ballasting techniques. Thus, during installation most of the structural members have one atmosphere of pressure on the inside and are exposed to the ambient pressure on the outside. After installation, the structural members are traditionally left void because it would be undesirable and normally not necessary to flood the structural members after the installation. Thus, most of the structural members that are immersed in seawater are exposed to the full ambient hydrostatic pressure during installation and during the life of the structure. For convenience and brevity, the terms xe2x80x9ctubularxe2x80x9d, xe2x80x9ctubularsxe2x80x9d, xe2x80x9ctubular membersxe2x80x9d, and xe2x80x9ctubular structuresxe2x80x9d will be used throughout this description to generally refer to the legs and truss members used in offshore structures. It is understood that the present invention is equally applicable to rectangular tubular shaped members or any other member shape such as a jacket launch cradle.
The hydrostatic pressure induces hoop compression in the tubulars. Also, the hydrostatic end forces induce axial compression in the tubulars. The stresses in the structural members caused by the hydrostatic pressure require extra wall thickness and often require that compression reinforcing rings be attached to the structural members at intervals along their lengths. These rings prevent instability and subsequent flattening of the tubular under the action of hydrostatic pressure. The deeper the water and the larger the member diameter, the greater is the expense related to hydrostatics. Ultimately, in deep enough water it becomes impractical to use void members, which means that extra buoyancy must be provided higher up in the structure to float the structure. In short, the design of the structure for hydrostatics becomes increasingly expensive with increasing depth and finally becomes prohibitive.
Another approach to the hydrostatic problem has been considered, but it has been rejected because of concerns over safety. If the members that are most affected by hydrostatics were filled with compressed air in the fabrication yard, then after installation the internal and external pressures acting on the tubulars could be balanced, thereby eliminating the hydrostatic problem. However, to satisfy concerns for safety these tubulars would have to be designed and rated as pressure vessels, which makes the idea too costly to use.
An alternative approach to the hydrostatic problem has been disclosed in U.S. Pat. No. 5,636,943. The proposed method stipulates that during fabrication of the offshore structure, a chemical of the required properties is placed on the inside of the structural members that are to be hydrostatically equalized during installation. The chemical is supplied in each member in the quantity required to generate a volume of gas that will create a pressure on the inside of the member that will be equal to the ambient pressure on the outside of the member when the structure is in its final in-place position. Such a member is said to be hydrostatically equalized. The chemical reaction that generates the gas is initiated by a device that is activated by differential pressure between the inside and outside of the member. In the preferred embodiment, the initiation devices are set at differential pressures that will initiate the gas generation reaction as the structure descends to its in-place position during installation. Thus, the members are not pressurized during fabrication onshore or during installation when the members are above the surface of the ocean, which means that personnel are never exposed to any danger from a pressurized unrated tubular. In fact, the differential set pressures can be chosen so that the tubular members are not exposed to net internal or external pressure that would control the design of the member. Inherent within this procedure is a lack of monitoring and control of the actual pressure generated within the tubular. Additionally, as generally described in the preferred embodiments there is an absence of any procedure to compensate for a lack of internal pressure where defective activation devices or defective chemical gas generating packages may exist.
The invention addresses the above need. What is provided is an apparatus and method for pressuring tubulars that does not expose personnel to any danger from a pressurized, unrated tubular. During the fabrication of an offshore jacket, compliant tower, or subsea template, tubular members are fabricated using normal practice such that each tubular member will contain an internal pressure of one atmosphere pressure. For selected members that are to be subsequently hydrostatically equalized, hardware means as necessary to permit subsea pressurization and isolation of the tubular internal pressure are installed while the structure remains in an onshore fabrication facility. Once completed, the structure is transported to an installation site, placed in the seawater, and positioned for lowering to a final position. The tubular members that have been selected to be hydrostatically equalized are connected to an umbilical line. Once connected, an isolation valve is opened, allowing compressed gas to be pumped into the tubular member until an internal pressure equal to or greater than the ambient pressure surrounding the brace at it""s final location is reached. After the required pressure is achieved, the isolation valve is closed to maintain brace internal pressure until such time as the member reaches its final location and final ambient pressure. The procedure is repeated until all selected tubular members have been hydrostatically equalized. At this point the structure may be safely installed at its final location. Thus, the tubular members are not pressurized during fabrication onshore or when the members are above the surface of the ocean.